A wind turbine blade typically comprises a shell which provides the blade with its aerodynamic properties and a load-carrying spar located inside the shell. The shell and the spar can be manufactured as separate members which are subsequently joined, e.g. by use of a polymer-based structural adhesive. For large blades a large amount of adhesive is applied, typically in the order of several hundred kilograms per blade, and the process is limited by the pumping capacity of the application equipment. The relatively long application time, due to the large amounts of material, means that at least some regions of the adhesive may react with the surroundings before joining which results in film formation at the surface. This may lead to regions with a so-called “kissing bond”, i.e. a weak bonding.
Another potential cause of areas with weak bonding is if the geometries of the surfaces to be joined do not completely match. This may result in local distances between the two surfaces being so large that air pockets appear in the adhesive afterwards. This potential cause is particularly related to surfaces with large curvature since the curvature makes it harder to obtain an exact desired geometry.
Weak bonding areas, such as air pockets, may result in a lower stability against buckling, and furthermore they increase the risk of crack initiation and growth.
Furthermore, the adhesive is applied and the members are joined while the shell is still in the mould, and the mould is therefore occupied for longer than the actual manufacturing of the shell itself.
The quality of the bonding is checked afterwards e.g. by ultrasound, and if it is found unsatisfactory, the bondline has to be improved by drilling holes in the shell and injecting additional adhesive after demoulding of the blade to ensure that the final bonding is satisfactory. The need for this additional manufacturing step leads to a less efficient manufacturing method than desired.
Hence, an improved method of manufacturing a wind turbine blade would be advantageous, and in particular a more efficient and/or reliable manufacturing method would be advantageous.